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If the engine runs OK under power when launching down the driveway *G* (I used to launch the race car up the driveway into the garage...), then likely you have a vacuum leak which is destroying your idle and low-rpm torque, but once the engine starts getting fuel from the main jets and the vacuum drops, like under load, things mellow out...

Check out your hose connections and for obvious gasket vacuum leaks.

Since you're running the E303 roller cam, which has 220 degrees duration @ .050" lifter rise, some idle reversion is to be expected. This could be the cause of the symptoms you describe. Depending on your CR and trans choice, idle quality could be problematical. Often, a secondary idle circuit helps this but is not possible on an Edelbrock, to my knowledge...

Describe the engine CR, trans type and converter stall speed, if auto...

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